1. Field of the Invention
The present invention relates to a method of measuring a gas concentration for measuring any one of concentrations of NOx, NO.sub.2, NO, and O.sub.2 by using a sensor element utilizing a metal oxide semiconductor as a sensing element.
2. Prior Art Statements
Usually, as a method of measuring NOx concentration in a gas to be measured including NOx, such as a combustion exhaust gas from an incinerator, a method for sampling a gas to be measured including NOx concentration in for example a gas duct, and measuring NOx concentration in the thus sampled gas by using an optical measuring apparatus is known. However, the optical measuring apparatus mentioned above is very expensive. Moreover, since it needs a gas sampling, there is a drawback such that a responsibility becomes worse.
In order to solve the drawback mentioned above, a semiconductor sensor of a gas duct direct insertion type has been used, recently. For example, Japanese Patent Laid-Open Publication No. 6-222028 (JP-A-6-222028) discloses a NOx sensor comprising a sensing portion made of a predetermined oxide of perovskite type, and a conduction measuring portion for measuring a conductivity of the sensing portion.
However, even in the semiconductor sensor of a gas duct direct insertion type mentioned above, any countermeasure for an influence of O.sub.2 and CO included in a gas to be measured with respect to a NOx measurement value is not taken. Moreover, a resistance of the sensing portion is normally varied corresponding to an amount of NOx(NO.sub.2 +NO)) i.e. a concentration thereof. However, in the case that an amount (concentration) ratio between NO.sub.2 and NO is varied, i.e. in case that a partial pressure ratio between NO.sub.2 and NO is varied, a resistance measured by the sensing portion is varied even if an amount of NOx is constant. Therefore, the semiconductor sensor mentioned above can not sense only NOx selectively. Accordingly, the semiconductor sensor of a gas duct direct insertion type mentioned above has a drawback,such that a NOx concentration in a gas to be measured can not be measured in a highly precise and selective manner, even if it is inexpensive and has an excellent responsibility as compared with the optical measuring apparatus.
On the other hand, in Japanese Patent Laid-Open Publication No. 7-20075 (JP-A-7-20075), there is disclosed a method of measuring a gas concentration in which concentrations of respective gas components are measured by using a predetermined formula utilizing detectors the number of which is equal to that of gas components to be measured. However, if the method mentioned above is applied to a NOx measurement, it is not possible to completely eliminate an influence of O.sub.2 with respect to a NOx measurement value, since this method uses a linear formula.